The Christmas Island Hawk-Owl

By Phosphate Resources in Environment

Phosphate Resources is committed to ensuring the Island’s unique flora and fauna is maintained for future generations. We are pleased to be supporting a conservation project for the Christmas Island hawk-owl coordinated by Range to Reef Environmental. This collaborative project with the Christmas Island District High School aims to evaluate the conservation value of artificial nest-boxes for this important species.

Figure 1. The Christmas Island Hawk-owl (Ninox natalis)

Figure 1. The Christmas Island Hawk-owl (Ninox natalis)

The Christmas Island hawk-owl (Ninox natalis) is Christmas Island’s only recorded owl species. It was described by Lister in 1888 and has been recognised as an individual species since 1998 (Norman et al. 1998). It is a nocturnal bird about the size of a common pigeon, and males and females appear similar, being easily recognisable by their barred brown plumage (Threatened Species Scientific Committee 2016; Figure 1). Christmas Island hawk-owls have a distinctive boo-book call and can often be heard in the evening.

This species is one of 11 owls found in Australia, occurring only on Christmas Island. The hawk-owl is widespread, occupying all major habitat types from closed canopy primary rainforest to marginal (disturbed and revegetated) land (Gibson-Hill 1947; Hill & Lill 1998). Despite being widespread across the island, hawk-owls appear to have a relatively small overall population. Estimates vary (Table 1), but the current official figures (approved July 2016) suggest there are fewer than 1,000 mature individuals (Threatened Species Scientific Committee 2016; after Garnett et al. 2011).

Surveys by Parks Australia indicate that the population is stable and that there is no immediate concern for its conservation (Woinarski et al. 2012, in Misso & West 2014). However, some studies have reported a potential overestimation (Low & Hamilton 2013) or decline (Morcombe 2016) in the hawk-owl’s abundance.

This uncertainty around the hawk-owls’ population status is due to the use of different survey designs, methodologies, and survey efforts in past population estimates. Further repeatable and consistent surveys are required to confirm the abundance and state of the hawk-owl. Today the species is formally classified as Vulnerable because of its small range on a single island, its susceptibility to effects of introduced taxa, and habitat loss (IUCN 2013; Threatened Species Scientific Committee 2016).

Table 1. Historical population estimates for the Christmas Island hawk-owl

Population Estimate Year Reference
‘Widespread and not rare’ 1947 Gibson-Hill 1947
10-100 breeding pairs 1975 van-Tets 1975
100 breeding pairs 1988 Stokes 1988
562 ± 105 pairs

820 – 1200 birds

1998 Hill & Lill 1998
1200 breeding individuals 2000 Garnett & Crowley 2000
1000 breeding individuals 2011 Garnett et al. 2011
10-100 pairs 2013 Low & Hamilton 2013
240-342 breeding individuals 2016 Morecombe 2016

Nesting habitat

There has been no conclusive demonstration of threats to the hawk-owl (Beeton et al. 2010), but one past impact may have been the loss of available nesting hollows in large rainforest trees in some areas. While the Christmas Island hawk-owl has been recorded throughout Christmas Island, the highest densities occur in primary rainforest habitat (Hill & Lill 1998; Morecombe 2016) and this is the only habitat where hawk-owl nests have been found (Hill & Young 1995, in Hill 2004).

Syzygium nervosum trees are characteristic of primary rainforest, forming approximately 18% of the forest canopy (Du Puy 1993). Unlike most other tree species in primary rainforest, it forms tree hollows which hawk-owls are believed to use for nesting (Hill & Young 1995, in Hill 2004). Ninox species are typically hollow-dependant, and because of the requirement for hollows, breeding opportunities for the Christmas Island hawk-owl may have declined in some areas due to the historical clearing of large mature Syzygium nervosum trees. For example, Hill and Lill (1998) estimated that prior to settlement, the Island could have had a carrying capacity of around 740 owl territories and suggested that since settlement, the total hawk-owl population could have theoretically decreased by 25% based on the 25% of forest that had been cleared on the Island since settlement (Olsen & Stokes 1989; Hill & Lill 1998).

In S. nervosum, natural hollows are thought to take many years to develop (likely several hundred years; Hill & Lill 1998 and reference therein), as is the case for many hollow-forming trees (e.g. Western Australia’s jarrah and marri forest trees need to be a minimum of 130 years old to be of use to hollow dependent fauna; Whitford & Williams 2002). Large historical clearing is likely to have removed some nesting trees used by hawk-owls, and this may have contributed to a reduction in hawk-owl abundance (Olsen & Stokes 1989; Hill & Lill 1998; Hill 2004).

It is interesting to note that while densities are highest in primary rainforest, hawk-owls still utilise other habitat types, such as rehabilitated and cleared land. Low and Hamilton (2013) detected at least seven hawk-owls utilising areas of mine lease (cleared land or areas of unrehabilitated coastal scrub), where some of these records were made in areas where the mine lease was not bordered by forest areas or land designated as National park.

Similarly, Hill & Lill 1998 recorded hawk-owl territories that included disturbed habitat (e.g. minefields surrounded or intersected by secondary vegetation, a major road, forest tracks, and an area of stockpiles). These areas are likely to provide good feeding opportunities for the hawk-owl. Hawk-owls have been observed foraging in fern fields (Hill & Lill 1998) and hunting along roads (Low & Hamilton 2013) and it has been suggested that hawk-owls concentrated in areas of high human habitation due to increased numbers of light-attracted prey (such as rats and geckos; Low & Hamilton 2013). It is highly likely that home range use by hawk-owls changes when the owls are nesting, and owls may move more widely once any chicks have fledged (Hill & Lill 1998).

Increasing nesting opportunities

Phosphate Resources Ltd recognised the potential for improving nesting opportunities in areas that were impacted by historical clearing, and in January 2017 initiated a pilot-study into the use of artificial nesting hollows. Artificial hollows have been used elsewhere in Australia to provide additional nesting habitat where hollow-bearing trees are limited or lacking. These artificial hollows usually take the form of nest-boxes which can be placed within natural vegetation or positioned upon man-made structures to supplement existing nesting habitat.

Nest-boxes have been used successfully by many bird species in Australia (Goldingjay & Stevens 2009), and the provision of nest-boxes has been instrumental in the recovery of numerous threatened species globally. For example, the provision of nest-boxes was critical to the successful re-establishment of the endangered Norfolk Island Boobook (Ninox novaeseelandiae undulata) in the Tasman Sea (Olsen 1996). Likewise, chicks of the endangered Carnaby Black Cockatoo (Calyptorhynchus latirostris) have fledged from nest-boxes provided in urban environments of Western Australia.

The major aim of the pilot project is to establish the potential to increase the availability of nesting habitat for the hawk-owl through the provision of artificial nesting hollows in disturbed areas of Christmas Island. In its initial stages, success will be defined when there is evidence of nest-box use by hawk-owls. While there is no guarantee hawk-owls will use the nest-boxes, if they are occupied, several questions can be answered or explored including:

  • Will hawk-owls use artificial hollows?
  • Is there any evidence of nesting or fledgling success arising from artificial hollow use?
  • Do other species use the nest-boxes on Christmas Island?
  • Is there a difference in nest-box use across different habitat types, designs, and/or positions?
  • What is the breeding biology of hawk-owls?

The final question is particularly important because detailed studies into the breeding biology of the hawk-owl have not been completed. Information on the timing and extent of the breeding season, the number of eggs laid, and fledgling success rates will be important for improving knowledge on the species’ ecology, and for the ongoing management of hawk-owls on Christmas Island.

A collaborative approach

PRL commissioned Perth-based environmental consultants Range to Reef Environmental (R2R) to implement the project. R2R designed the nest-boxes with input from Parks Australia staff and expert ornithologists with experience working on the hawk-owl or on nest-box projects. R2R then established a partnership with the Christmas Island District High School to build the boxes. School students made thirty nest-boxes as part of their Technology Education curriculum (Figure 2), which were installed in areas of disturbed habitat in July, 2017 (Figure 3).

Figure 2 – Students and teacher Brad Boucher constructing nest-boxes for the hawk-owl.

Figure 3 – A hawk-owl nest-box being installed in an area of disturbed forest.

Figure 3 – A hawk-owl nest-box being installed in an area of disturbed forest.

Monitoring and beyond

Students of the years 7-10 will now undertake a ‘citizen-science’ program under the supervision of R2R. Students will shortly commence monitoring the nest-boxes in conjunction with environmental staff from Christmas Island Phosphates and R2R, recording what species (if any) use them. Some boxes are also being monitored by cameras (Figure 4). The boxes will remain in place for at least one year, and the information obtained from the nest-box pilot study will be analysed to determine whether hawk-owls on the Island will use artificial nesting hollows provided within their natural environment.

If the nest-boxes are successful, there may be the possibility to rollout the project on a wider scale in the future, with nest-box construction potentially becoming an annual classroom activity that promotes environmental stewardship and provides a valuable contribution to the conservation outcomes for this species.

Further updates will be posted to the PRL Facebook page and blog. A copy of the hawk-owl nest-box project flyer can be downloaded here.

Figure 4 – Hawk-owl nest-box being monitored by a motion activated camera.

Figure 4 – Hawk-owl nest-box being monitored by a motion activated camera.